"(C-a <Tab>)
Move the input focus to the next region. This is done in a cyclic way so that the top region is selected after the bottom one. If no subcommand is given it defaults to `down'. `up' cycles in the opposite order, `top' and `bottom' go to the top and bottom region respectively. Useful bindings are (j and k as in vi)
bind j focus down
bind k focus up
bind t focus top
bind b focus bottom"

I've bound "C-a p" to "focus up" and "C-a n" to "focus down"... And they do exactly what is said: they switch to the top/bottom region, but NEVER to the region on the right or on the left and they never go back to an lower/upper region.
On the contrary, the default C-a <Tab> focuses alternatively on each region of the whole list of regions, going up/down/top/bottom if necessary and cycling through the list...

As a conclusion, that means that, contrarily to what the documentation says:
1. "focus <argument>" does not follow a cycle process;
2. "focus" with no argument does not behave by default as "focus down"...

So either the documentation or the implementation is wrong...

This is particularly annoying, since I'm trying to bind a key to a command that would cycle through the whole list of regions, but in reverse order (in order not to have to go through a cycle of X (>>2) "C-a <Tab>" to reach the previous region), but such a command actually doesn't exist :[